The present invention relates generally to power amplifiers. More specifically, the present invention relates to a distributed power amplifier in a monolithic microwave integrated circuit. Monolithic microwave integrated circuits employ "distributed amplifiers" to provide extremely broadband amplification. Distributed amplifiers have a flat frequency response from essentially direct current all the way to as high as tens of Gigahertz. Edward L. Ginzton, et al. reported the use of the distributed amplifier, also referred to as a traveling wave amplifier, in a paper entitled "Distributed Amplification" published August, 1948 in the Proceedings of the IRE, hereby expressly incorporated by reference for all purposes.
Since the introduction of the distributed amplifier concept, there have been many improvements. For example, U.S. Pat. No. 4,446,445 entitled Singly Terminated Push-Pull Distributed Amplifier, issued May 1, 1984 and U.S. Pat. No. 4,540,954 entitled Singly Terminated Distributed Amplifier, both hereby expressly incorporated by reference for all purposes, illustrate improvements in distributed amplifiers.
FIG. 8 is a block diagram of a conventional distributed amplifier 800. Distributed amplifier 800 includes an input transmission line 802, an output transmission line 804, a plurality of equalizing transmission lines 806.sub.i, a plurality of FET devices 808.sub.i, an input transmission line terminating impedance 810, and an output line terminating impedance 812. FET devices 808.sub.i include a gate-source capacitance and a drain-source capacitance. FET devices 808.sub.i couple input transmission line 802 to output transmission line 804. FET devices 808.sub.i are spaced uniformly along the transmission lines with the gates coupled to input transmission line 802, the drains coupled to output transmission lines 804, and the sources connected to ground. The gate-source capacitances periodically load input transmission line 802, while the drain-source capacitances periodically load output transmission line 804. The drain-source capacitances of the FET devices 808.sub.i are not always sufficient to equalize the characteristic impedances and velocities of the transmission lines, therefore equalizing transmission lines 806.sub.i are sometimes put in series with the drain-source capacitances. It is well-known to model the transmission lines as a series of periodic series inductances and shunt capacitances. Thus, input transmission line 802 includes input inductances 814.sub.i, associated with FET gate to source capacitances, and output transmission line 804 includes output impedances 816.sub.i, associated with FET drain to source capacitances.
FIG. 9 is a schematic representing a model for an equivalent circuit of distributed amplifier 800 shown in FIG. 8. This model results from replacing the plurality of FET devices 808.sub.i with a current source 902.sub.i and a capacitance 904.sub.i (the source/drain capacitance) for the output transmission line side. For the input transmission line side, the gate source capacitance is modeled by capacitance 906.sub.i. The equivalent circuit incorporates a classic low-pass filter design.
FIG. 10 is a block schematic diagram illustrating conventional use of a large inductive element 950 to isolate a drain voltage source from low-pass network structures in a distributed amplifier, such as distributed amplifier 800 in FIG. 8. To operate distributed amplifier 800, it is necessary to bias the drains of the plurality of FET devices 808.sub.i as well-known. Inductive element 950 has a relatively large, at frequencies of interest, reactance that couples a bias voltage from a voltage source 952 to the drains of the plurality of FET devices. The reactance of inductive element 950 is large enough to isolate voltage source 952 from the distributed amplifier network so as to preserve the frequency characteristics, the low-pass filter characteristics, of distributed amplifier 800.
Inductive element 950 carries relatively large currents, typically on the order of one amp or more, therefore an integrated circuit including a distributed amplifier like distributed amplifier 800 employs wide metalization lines to adequately handle this current. The wider the metalization lines become, associated parasitic capacitance of the lines, which comprise the induction element 950, becomes a more significant factor. This associated parasitic capacitance limits wideband practicality of the conventional distributed amplifier.